Microcellulose (also named as e.g. level-off DP cellulose and micro crystalline cellulose) is a versatile product in many industrial applications, e.g. in food, pharmaceutical, cosmetics, paper and board and many other applications. Microcellulose can also be used in the production of derivatives of microcellulose, such as viscose cellulose, CMC, nanocellulose and various composite products.
Several methods have been proposed in the patent literature for producing microcellulose.
U.S. Pat. No. 2,978,446 describes the production of level-off DP (degree of polymerization) cellulose by acid hydrolysis and mechanical treatment. Cellulose is hydrolyzed with boiling in 2.5 normal hydrochloric acid (HCl). Acid concentration is thus 9% and temperature about 105° C. The consistency of the pulp and the amount of added acid are not specified. The method requires subsequent to acid hydrolysis vigorous mechanical disintegration in aqueous medium. It is obvious that high dosage of acid and extensive mechanical treatment have prevented the development of any reasonable production economy.
U.S. Pat. No. 3,278,519 describes a similar method for producing level-off DP cellulose by hydrolyzing cellulose either with 2.5 normal HCl at 105° C. or with 0.5% HCl at 250° F. (121° C.). The consistency of the pulp and the amount of added acid are not specified. This method also describes a harsh energy-consuming mechanical treatment.
U.S. Pat. No. 3,954,727 discloses a method for producing microcrystalline cellulose by hydrolyzing cellulose with dilute sulphuric acid at a temperature of from 120 to 160° C. The dilute sulphuric acid to which the cellulose is added has a concentration of 1% and the cellulose-acid mass has a concentration of 5%. Thus, the consistency of the pulp is low, the heating demand is therefore high and the amount of acid based on the dry weight of the cellulose becomes high. This makes the process more expensive and more complicated.
U.S. Pat. No. 7,037,405 describes a method, in which raw pulp material is contacted with acid and heated at elevated temperature and then treated mechanically. A suitable acid concentration is mentioned to be 1-5% of the mixture, a suitable pulp consistency 3-50%, a suitable temperature range 80-120° C. and a suitable reaction time 30 min-4 h. After acid hydrolysis the pulp mixture is treated mechanically for disintegration of the fibres. Preferably the mechanical disintegration process step shears the crystalline cellulose particles into micron size ranging form about 1 to 10 micron size. The process of U.S. Pat. No. 7,037,405 suffers of complicated production process. Mechanical disintegration step is required after acid hydrolysis. This stage requires in production costly refiner unit and high refining energy of 5-100 kWh/ton.
U.S. Pat. No. 6,228,213 discloses a process for producing microcrystalline cellulose by adding an acid solution to cellulose and feeding the cellulose and acid solution through an extruder, wherein the cellulose undergoes acid hydrolysis and forms microcrystalline cellulose. The temperature of the extruder barrel during the hydrolysis is from 80 to 200° C. Due to the temperature of the extruder and the pressure created by the die or screw of the extruder, the cellulose melts in the extruder, which allows for more intimate contact between the cellulose and the acid. The compression ratio of the extruder screw is between 1.5:1 and 3:1, preferably about 3:1. Disadvantages with extruders are that they are expensive, the maintenance costs are rather high, and they require a high mechanical energy input, by estimation about 150 kWh or over per dry ton cellulose.
U.S. Pat. No. 5,543,511 describes the production of level-off-DP cellulose using partial hydrolysis with oxygen and/or carbon dioxide at 100-200° C.
U.S. Pat. No. 4,427,778 describes the production of level-off-DP cellulose by enzymatic hydrolysis.
Acid hydrolysis is also used in the dissolving pulp production, e.g. in acidic bisulphite cooking and as a pre-hydrolysis step in the kraft process. Acidic bisulphite cooking is described in e.g. Rydholm, S. E., Pulping Processes, pp. 649 to 672. U.S. Pat. No. 5,589,033 describes a pre-hydrolysis kraft process of lignin-containing cellulosic material (i.e. wood chips) at 100-160° C. for softwoods and at 120-180° C. for hardwoods for 10-200 min. Neutralization and alkaline kraft cooking follows the pre-hydrolysis step. The final product is a pulp with high alpha cellulose purity and can be used as dissolving pulp. Dissolving pulp has a fibrous structure with fiber length of typically 0.5 mm to 2.5 mm dependent on wood raw material. Thus, the physical dimensions are much larger than microcellulose.
In the prior-art microcellulose manufacturing processes high amounts of chemicals such as acids and neutralization chemicals are used.
Chemical recovery of kraft pulp mills are based on converting sodium and sulfur into active cooking chemicals by a multi-step recovery process including black liquor evaporation, recovery boiler and caustisizing steps. The dissolved substances are burned in a recovery boiler whereby also energy is produced. The recovered active cooking chemicals are in sodium hydroxide and sodium sulfide or sodium hydrogen sulfide form Balancing the ratio of sodium and sulfur is very essential in these recovery processes. Typically chemical recovery systems of softwood kraft pulp mills experience an excess of sulfur and therefore have a challenge to bleed out sulfur from the closed cycle. Sulfur removal can take place by taking out glauber salt (i.e. sodium sulfate salt) from recovery boiler. The problem here is that also sodium will simultaneously be removed and therefore this alternative is inefficient and expensive as addition sodium (e.g. in form of make-up caustic, NaOH) needs to be added to the recovery cycle.
In view of the above described processes for producing microcellulose there is a need for an even more efficient and economical process. In addition, the kraft pulp mills have a need to better balance the sodium-sulfur ratio.